1. Field of the Invention
This invention relates generally to rotary power transmission systems, and more particularly to fluid drive transmissions utilizing a fluid coupling and a lockout clutch to lock the impeller and output turbine of the fluid coupling together under predetermined operating conditions. Even more particularly, the present invention relates to the use of such a fluid coupling and lockout clutch in combination with a speed transmission usable for high torque applications such as those encountered in transmitting power from the engine to the driving wheels of a tractor.
2. Description of the Prior Art
Speed transmissions are well known and have been used for some time in tractor applications, with a typical speed transmission of the hydromechanical type being described in U.S. Pat. No. 4,138,907, which is assigned to the same assignee as the present invention. In addition, the use of fluid drive lockup clutches is also known, particularly in conjunction with torque converters of automobile transmissions and the like, as described in U.S. Pat. Nos. 3,693,478, 3,809,956; 3,861,241; 4,033,202; 4,056,177; and 4,152,984. However, no attempt has been made to utilize a lockup clutch in a fluid coupling, particularly in a fluid coupling and speed box utilized as a transmission to apply power from an engine to the driving wheels of a tractor.
Background patents further include but are not limited to Pat. Nos. 3,390,594; 3,949,847; 4,056,177; and especially 4,131,184 and 4,208,929.
In a typical tractor transmission, such as for example, the one described in the aforesaid U.S. Pat. No. 4,138,907, the engine is disclosed coupled to a hydromechanical transmission which is, in turn, coupled to a variable ratio machanical transmission via a master clutch. The output of the variable range mechanical transmission drives the driving wheels of the tractor, and the hydromechanical transmission is generally coupled directly to the flywheel of the engine, but in some instances a fluid drive may be used to couple the engine to the hydromechanical transmission.
While such systems have been used successfully in tractors and other vehicles, tractors employing a transmission that is directly coupled to the engine are difficult to control at low speed, and require that the master clutch be slipped when the tractor is moved slowly. Such clutch slipping results in wear and deterioration of the master clutch, and also in a bucking of the tractor. This bucking is not only unpleasant to the operator, but can be quite dangerous in many situations. For example, when a tractor is being hitched to a farm implement or trailer, an unfortunate buck of the tractor might give a discomfitting jolt to a person holding the hitch of the tractor in alignment with the hitch of the implement.
In an attempt to improve the controllability of tractors and the like at low speed, some manufacturers have utilized a fluid coupling between the engine and transmission, because a fluid coupling has a certain amount of slip which reduces the tendency of a tractor to buck and improves low speed control. However, permitting such fluid slip is detrimental in that it reduces the fuel efficiency of the tractor; thus, the design of the fluid coupling requires a trade-off between low speed operation and fuel efficiency, with greater slip favoring low speed operation at the expense of fuel efficiency and vice versa. Known teachings in the art lead away from our unique locking coupling in fact, because we recognize, as a decided departure form prior art, utility in an inordinately high-slipping fluid coupling and we realize, as a decided departure from the art, that particular utility by our novel design achieving the high slip.